The present invention pertains to aqueous dispersions of hydrogel nanoparticles and methods of making mono-disperse interpenetrating polymer network (“IPN”) nanoparticles. More specifically, the IPN nanoparticles are made from a first polymer that is formed by polymerizing a first mixture of monomers, a cross linking agent, and an initiator to form a first polymer nanoparticle. The first polymer nanoparticle is swollen with a second combination of monomers and cross linking agents, which is subsequently polymerized to form the mono-disperse IPN nanoparticles having unique thermally induced gelling properties. This thermally induced viscosity change, and in situ hydrogel formation allows for therapeutic medications to be mixed into a liquid form of the nanoparticles and be uniformly distributed in a solid hydrogel at an elevated temperature without being destroyed by harsh chemical processes typically used to solidify a liquid. Additionally, the solid hydrogel allows the controlled time release of medications. This invention also includes compositions and methods for the synthesis of the soft nanoclusters composed of several crosslinked IPN nanoparticles building blocks.
Generally, effective drug therapies usually require that: (a) a certain concentration level of a medication (called the therapeutic index) be maintained for; (b) a certain period of time. For example, a systemically administered medication may quickly elevate the concentration level of the medication, but many delivery systems have the potential to be inefficient and are accompanied with toxic side effects resulting from high doses. In contrast, systemic administration of controlled release formulations can accomplish both objectives with a more efficient delivery of the drug that may reduce side effects. Additionally, implantation of a drug delivery system may further improve the efficiency for utilizing a locally specific drug.
Generally, hydrogels are materials that absorb solvents (such as water), undergo rapid swelling without discernible dissolution, and maintain three-dimensional networks capable of reversible deformation. Hydrogels may be uncrosslinked or crosslinked. Uncrosslinked hydrogels are typically able to absorb water but do not dissolve due to the presence of hydrophobic and hydrophilic regions. Covalently crosslinked networks of hydrophilic polymers, including water soluble polymers, are traditionally denoted as hydrogels in the hydrated state. A number of aqueous hydrogels have been used in various biomedical applications, such as, soft contact lenses, and wound management. Additionally, hydrogels have been previously utilized as drug delivery systems, for example: U.S. Pat. No. 6,639,014 issued to Pathak, et al., on Oct. 28, 2003, titled “Multiblock Biodegradable Hydrogels for Drug Delivery and Tissue Treatment, (“the '014 Patent”); and U.S. Pat. No. 6,632,457 issued to Sawhney on Oct. 14, 2003, titled “Composite Hydrogel Drug Delivery Systems,” (“the '457 Patent”).
Hydrogels containing two IPN's are the subject of intensive investigation (Ilmain, F., Tanaka, T., Kokufuta, E. Nature (London, United Kingdom) 1991, 349, 400-1). This is because an IPN hydrogel usually exhibits properties that a hydrogel with the random co-polymerization of two monomers does not have. For example, the IPN of poly(acrylic acid) (“PAAc”) and polyacrylamide (“PAAM”) undergoes the volume phase transition driven by cooperative “zipping” interactions between the molecules which result from hydrogen bonding (Ilmain, F., Tanaka, T., Kokufuta, E. Nature (London, United Kingdom) 1991, 349, 400-1). In addition to the improved mechanical properties which usually come from the reinforcement between two interpenetrating networks, (Sperling, L. H. Adv. Chem. Ser. 1994, 239, 12). an IPN hydrogel can have a preferred direction for swelling by pre-stressing one of them (poly-N-isopropylacrylamide (“PNIPAM”)) before the gelation of the other one (polyacrylamide (“PAAM”)) takes place (Wang, C. J., Hu, Z. B., Chen, Y. Y., Li, Y. Macromolecules 1999, 32, 1822-1827). A well-designed hydrogel with an IPN structure shows an upper critical solution temperature without a volume change (Chen, L., Gong, J., Osada, Y. Macromolecular Rapid Communications 2002, 23(3), 171-174). Two polymer chain networks in an IPN gel can be sensitive independently to two different external stimuli. Such hydrogels have been employed for controlled drug delivery (Katono, H., Sanui, K., Ogata, N., Okano, T., Sakurai, Y. Polym J 1991, 23, 1179-1189; Katono, H., Maruyama, A., Sanui, K., Ogata, N., Okano, T., Sakurai, Y. J Control Rel 1991, 16, 215-227 273:464-472; Gutowska, Y. H., Bae, H., Jacobs, J., Feijen, Sung Wan Kim, Macromolecules, 1994, 27, 4167; Park, T. G., Choi, H. K. Macromol. Rapid Commun. 1998, 19, 167-172) The PNIPAM gel undergoes the volume phase transition at Tc=34° C. and has been used often as one of the components in an IPN gel (Hirotsu, S., Hirokawa, Y., Tanaka, T. J. Chem. Phys. 1987, 87, 1392; Wu, C., Zhou, S. Macromolecules 1996, 29, 1574; Benee, L. S., Snowden, M. J., Chowdhry, B. Z., Langmuir, 2002, 18, 6025; Routh, A. F., Vincent, B., Langmuir, 2002, 18, 5366-5369; Woodward, N. C., Chowdhry, B. Z., Snowden, M. J., Leharne, S. A., Griffiths, P. C., Winnington, A. L., Langmuir, 2003, 19, 3202-3211; Gao, J.; Frisken, B. J., Langmuir, 2003, 19, 5217-5222). Its phase transition temperature remains the same if the PNIPAM is incorporated in an IPN matrix. However, the random copolymerization results in shifting Tc depending on the hydrophilic/hydrophobicity of the co-monomer.
IPN microgel particles have been synthesized because they are more effective as delivery systems than macroscopic gels for agrochemical or medical applications (Bouillot, P., Vincent, B. Colloid and Polymer Science 2000, 278, 74-79). A comparison of the swelling behavior of the random P(AAc-co-Aam) particles and PAAc/PAAm IPN microgels has been made using temperature and pH as the triggers (Bouillot, P., Vincent, B. Colloid and Polymer Science 2000, 278, 74-79). Jones and Lyon, on the other hand, showed multiresponsive core-shell microgels that consist of a weakly interpenetrating polymer network core and a shell (Jones, C. D., Lyon, L. A. Macromolecules 2000, 33, 8301-8306). These microgels were prepared by precipitation polymerization at 70° C. in aqueous media. In the synthesis of the shell polymer, the collapsed particles serve as nuclei for further polymerization, thereby resulting in preferential growth of the existing particles over the nucleation of new ones (Jones, C. D., Lyon, L. A. Macromolecules 2000, 33, 8301-8306). The IPN nanoparticles of this invention are substantially free from the typical collapsed core and shell polymers described above.
A method to synthesize an IPN microgel of PNIPAM/PAAc is described herein. The Jones and Lyon's method (as described in (Jones, C. D., Lyon, L. A. Macromolecules 2000, 33, 8301-8306) was extended by lowering the reaction temperature below the lower critical solution temperature of the PNIPAM polymer and by controlling reaction time so that the reaction was stopped once the interpenetrating polymer network was formed at room temperature. The synthesis and light scattering characterization of these microgels, which displays the same Tc as the PNIPAM but shrinks less than the PNIPAM above Tc are shown. The semi-dilute aqueous solutions of the PNIPAM-PAAc IPN microgels exhibit unusually inverse thermo-reversible gelation. In contrast to polymer solutions of poly(NIPAM-co-AAc) that have the inverse thermoreversible gelation, the disclosed system can self-assemble into an ordered structure, displaying bright colors (T. Kato, M. Yokoyama, A. Takahashi, Colloid & Polym. Sci. 1978, 256, 15; M. Almgren, P. Bahadur, M. Jansson, P. Li, W. Brown, A. J. Bahadur, Colloid & Interface Sci. 1992, 151, 157; P. Alexandridis, T. A. Hatton, Colloidal Surfaces A: Physicochem. Eng. Aspects 1995, 96, 1-46; C. K. Han, Y. H. Bae, Polymer 1998, 39, 2809-2814; B. Jeong, S. W. Kim, Y. H. Bae, Adv. Drug Del. Rev. 2002, 54, 37-51).
The IPN nanoparticles aqueous dispersions possessing the sharp inverse thermo-gelation property tunable in a very narrow temperature range (1-2° C.) are useful in a number of biomedical applications that may include tissue engineering scaffold, bio-adhesive, cell culture matrix and controlled drug delivery. More specifically, the focus of this invention pertains to the advantages in the controlled drug loading and release aspects. Thermally responsive bulk gels are usually formed through free radical polymerization of monomers (R. Yoshida, K. Uchida, Y. Kaneko, K. Sakai, A. Kikuchi, Y. Sakurai, and T. Okano, Comb-type grafted hydrogels with rapid de-swelling response to temperature changes, Nature 374 (1995) 240; Y. H. Bae, T. Okano, and S. W. Kim, Thermo-sensitive polymers as on-off switches for drug release, Makromolecular Chemistry Rapid Communication 8 (1987) 481; T. Okano, Y. H. Bae, H. Jacobs, and S. W. Kim, Thermally on-off switching polymers for drug permeation and release, Journal of Controlled Release 11 (1990) 255; Y. H. Bae, T. Okano, and S. W. Kim, “On-off” thermocontrol of solute transport. I. Temperature dependence of swelling of N-isopropylacrylamide networks modified with hydrophobic components in water, Pharmaceutical Research 8 (1991) 531; Y. H. Bae, T. Okano, and S. W. Kim, “On-off” thermocontrol of solute transport. II. Solute release from thermosensitive hydrogels, Pharmaceutical Research 8 (1991) 624). Bulk gels have a permanent structure of covalently bonded polymer chains. In contrast, the IPN nanoparticle networks discussed here have a thermally reversible structure, and the physical bonds between the neighboring particles can be turned on of off by switching the temperature above or below the gelation temperature. Furthermore, a drug molecule is usually loaded in a bulk gel by either mixing drug with monomer, initiator and crosslinked, or allowing a bulk gel to swell to equilibrium in a suitable drug solution (Y. H. Bae, T. Okano, S. W. Kim, Hydrogels, Swelling and Drug Loading and Release, Pharma. Res. 9 (1992) 283). The first approach may suffer from the possibility of side reaction that can damage the drug, while the second approach may exclude large molecules, like proteins, from the gel network (L. E. Bromberg, E. S. Ron, Temperature-responsive gels and thermogelling polymer matrices for protein and peptide delivery, Adv. Drug Deliv. Rev. 31 (1998) 197). In contrast, a drug molecule can be mixed into the nanoparticle dispersion at room temperature. At body temperature (37° C.), the particles were bonded by physical bonds to form a gel to allow the drug slowly diffusing out of the nanoparticle network. Because there is no chemical reaction involved, the drug molecule could be entrapped nanoparticle network safely.
Additionally, the IPN microgels could be used as a special building block to build the mono-dispersed soft nanoclusters due to the strong inter-particles hydrophobic attraction even in a relatively dilute environment at T>34° C. In this disclosure, the mono-disperse soft nanoclusters were synthesized by zero-distance covalent bonding a certain number of microgels composed of poly-acrylic acid (“PAAc”) and poly(N-isopropylacrylamide) (“PNIPAM”) interpenetrating networks (“IPN”). The kinetics of the nanoclusters formation was obtained by measuring the particles hydrodynamic radius (“Rh”) change as a function of the reaction time. Individual nanocluster was characterized by both dynamic and static light scattering, and compared to its building block IPN microgels. The number of IPN microgels in each nanocluster can be statistically manipulated through controlling the reaction duration. The nanoclusters undergo a reversible volume phase transition in response to the temperature changes, but have no obvious sensitivity to pH. In contrast to its building block IPN microgels, the nanoclusters exhibited an enhanced de-swell ability upon temperature increase. This novel soft nanomaterial possesses unique pomegranate-like architecture and may have great potential for injection drug loading and delivery applications.
A preferred embodiment of the present invention pertains to a composition and process for synthesizing mono-disperse nanoparticles having the interpenetrating structure of poly(N-isopropylacrylamide) and poly(acrylic acid). The preferred IPN nanoparticles microgels have physically crosslinked IPN's and a property of reversible gel formation. The physical structures of specific crosslinked mono-disperse nanoparticle IPN microgels allow these materials to be utilized as timed drug release carriers. One of ordinary skill in the art can utilize the principles described herein to design and fabricate other hydrogel nano/micro-particles water dispersions possessing having similar inverse thermoreversible gelation properties. The preferred nanoparticles include but are not limited to: poly(acrylic acid)/poly(N-isopropylacrylamide) IPN nanoparticles; poly(acrylic acid)/hydroxypropylcellulose IPN nanoparticles; dextran/poly(N-isopropylacrylamide) IPN nanoparticles and dextran/hydroxylpropylcellulose IPN nanoparticles.